Being a member of many miles/points dining versions of RN, I have lots of credit cards registered. As it's convenient to be able to move whichever card I want to use around sometimes, I just leave old (i.e. dead/expired) cards as placeholders. Recently, I've had a flurry of emails from various RN dining sites of this format:

I don't know why they say "due to recent changes", because I doubt anything has changed. I don't know why they are bothering, as disallowing RN to use a credit card that doesn't exist has no effect on anything. They haven't actually removed them from my profiles, so the old cards still seem to do their job as placeholders to ensure that I always have 1 card registered per site. Maybe they will eventually be removed? But then what happens if that was your only registered card?

I don't think they're referring to expired cards. They no longer ask for expiration date (like they did years ago), so I don't think they can even know the cards are expired. I think it's something else (but I'm not sure what).

Is there anything "non-standard" about the specific cards they emailed you about? Like, for example, did they used to allow something other than MC/Visa/Amex/Discover (since that's the only 4 they now accept for "add card")?

Even if it's not simple, there's a lot of information that can be gleaned from the first digits of a card number. They can figure out, for example, which bank issues it, and if they hate Bank Zebra for some reason, they can disallow all cards issued by Bank Zebra. They can even get more specific, I think, if they wish.

What are these emails titled? I don't think I've gotten any such emails (and, btw, I have tons of expired cards in my various RN accounts).

Oh, I just noticed you seem to be based in Australia (you show you Location as MEL). Maybe they no longer accept cards issued in certain countries (which they can figure out from the number itself), and that's what they disabled?

They are titled "Important changes may affect how you earn rewards".

I've only received a few of them and there's no obvious pattern, except that they are very much expired. (Not by date, by not being valid at all.) Certainly doesn't seem to be anything to do with the first digits of a card number. For example, I had an Amex which I changed to a different type. It was cancelled and a new one issued. The only thing which changed was the last 4 digits. The old version was one of the cards RN emailed me about. The new version was accepted by RN.

Looking up old details, they are all Amex cards. I had thought one of them wasn't, but it was a co-branded one from about 5 years ago that I'd forgotten. They may be non-US ones. (I have both US and non-US cards and some have shared last 4 digits, so can't be sure!)